public class Solution21 {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2){
        ListNode head = new ListNode(-1);
        ListNode p = head;
        ListNode p1 = list1;
        ListNode p2 = list2;
        while (p1 != null && p2 != null){
            if(p1.val < p2.val){
                p.next = p1;
                p1 = p1.next;
            }else {
                p.next = p2;
                p2 = p2.next;
            }
            p = p.next;
        }
        if(p1 != null){
            p.next = p1;
        }
        if(p2 != null){
            p.next = p2;
        }
        return head.next;
    }
}
